JP2007273682A - Heat sink and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat sink of copper or copper alloy on which a semiconductor substrate is mounted by solder jointing, capable of stably preventing formation of voids, caused by Cu diffusion when Pb-free solder is used and defective solder wettability, for proper bonding characteristics to a power module case. <P>SOLUTION: In a semiconductor substrate 7 is mounted by solder jointing 2 on a heat sink 1 of copper or copper alloy, Ni plating is applied on a surface, on which the semiconductor substrate is mounted (substrate-mounting surface) and on the surface (rear surface) on the opposite side of it. The fluctuation width in the Ni plating thickness at five points, including the proximity of an end surface and a central part of plate surface, is 2.0 μm or smaller, on the substrate mounting surface and the rear surface, as well. Those, especially having an end face on which no Ni plating is applied, become appropriate objects. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a heat radiating plate made of copper or a copper alloy bonded to a substrate on which a semiconductor element is mounted (referred to as “semiconductor substrate” in this specification), and the plate surface is subjected to Ni plating.

  As means for efficiently dissipating heat generated from a semiconductor element, means for mounting a semiconductor substrate on a “heat radiating plate” made of a material having good thermal conductivity is widely adopted.

  FIG. 1 schematically shows a configuration example of a semiconductor module in which a semiconductor substrate is mounted on a heat sink. A copper pattern 4 is formed on the surface of an insulating substrate 5 made of a ceramic such as alumina or aluminum nitride, and a conductor layer 6 made of a copper plate or the like is formed on the opposite surface. The insulating substrate 5, the copper pattern 4, and the conductor layer 6 are integrated to constitute the semiconductor substrate 3. On the surface of the semiconductor substrate 3 on which the copper pattern 4 is formed, for example, a semiconductor element 7 is mounted via a solder layer 2. A lead wire 8 made of a conductive material such as Al is attached between the copper pattern 4 and the semiconductor element 7 as necessary to constitute a circuit. On the other hand, the surface of the semiconductor substrate 3 on which the conductor layer 6 is formed is bonded to the heat sink 1 via the solder layer 2.

  Since the insulating substrate 5 requires a coefficient of thermal expansion as small as that of the semiconductor element, for example, the insulating substrate for power semiconductor is made of ceramics. On the other hand, since the heat sink 1 is not directly joined to the semiconductor element, it is not required to have a small thermal expansion coefficient equivalent to that of the semiconductor element. Rather, thermal conductivity is given priority. For this reason, a plate of copper, copper alloy, aluminum or aluminum alloy having good thermal conductivity is mainly used as the heat sink. In addition, since this heat sink causes "warping" due to the difference in thermal expansion with the substrate during soldering, there is a means to warp in the reverse direction by press molding in advance for the purpose of offsetting this warpage. May be taken.

A heat sink made of copper or a copper alloy is often used after being plated with Ni. The reason is as follows.
(I) It is desirable to prevent diffusion of Cu atoms into the solder layer on the solder joint surface. In particular, in Pb-free solder (for example, Sn—Ag series), voids due to diffusion of Cu are likely to occur, which causes generation of solder cracks and deterioration of heat dissipation. When Ni plating is applied, diffusion of Cu into the solder layer is suppressed.
(Ii) In many cases, the rear surface of the semiconductor substrate mounting surface is attached to an aluminum radiating fin, and in that case, if an oxide film is formed on the surface of the radiating plate, the heat dissipation is reduced. When Ni plating is applied, the corrosion resistance is improved and the formation of such an oxide film is suppressed.

  In heatsink applications, if the press is performed after Ni plating, the end face is exposed by press punching, the corrosion resistance is deteriorated, and press scratches are likely to occur. Conventionally, Ni plating is applied after press punching or warping.

JP 2000-311973 A Japanese Patent Laid-Open No. 2000-219996

  However, conventionally, even in a heat sink made of copper or copper alloy plated with Ni, void formation due to Cu diffusion when using Pb-free solder may be a problem, and the expected effect of Ni plating is In some cases, it was not fully demonstrated. Further, even when ordinary Pb-containing solder was used, the solder wettability was sometimes reduced. Furthermore, when the heat sink is used by being fitted in the resin case of the power module, the end face of the heat sink is smoothed by Ni plating, so that there may be a case where good bondability with the case cannot be ensured.

On the other hand, when Ni plating is performed before press punching, deterioration of corrosion resistance and generation of surface scratches due to exposure of the end face, which was not a problem with mere punching with a bus bar or electrode terminal material having a thickness of less than 1 mm as in Patent Document 2, However, a heat sink having a thickness of 1 mm or more may have an adverse effect.
An object of the present invention is to solve these problems while suppressing costs.

The inventor has investigated many samples in order to investigate the cause of the formation of voids in the solder layer despite the Ni plating. As a result, it was found that in the portion where the Ni plating thickness is thin, Cu diffusion cannot be sufficiently suppressed, so that Cu eventually reaches the solder layer and a void is generated. Therefore, when measures were taken to sufficiently increase the Ni plating thickness, generation of voids due to Cu diffusion was avoided. However, even when warping is applied by a press, the amount of warping previously applied fluctuates and often deviates from the target amount of warping. After various investigations about the cause, heat and electrodeposition stress applied during the plating process, difference in plating thickness between the center and end face of the heat sink, and electroplating after press warping, etc. As a factor, it has been found that control of the difference in the plating thickness is particularly important. That is, simply increasing the plating thickness does not provide a measure for fluctuations in the amount of warping. Further, it is not preferable to make the Ni plating thick because it leads to an increase in cost.
Furthermore, it has been found that the decrease in solder wettability can be caused by non-uniform Ni plating thickness.

  Therefore, as a result of further detailed research, it was found that the problem of void generation can be solved while suppressing costs in a heat sink having a small variation in Ni plating thickness. At the same time, it was confirmed that the solder wettability was greatly improved.

That is, according to the present invention, in a heat sink made of copper or copper alloy having a thickness of 1 to 5 mm for mounting a semiconductor substrate by solder bonding, the surface on which the semiconductor substrate is mounted (substrate mounting surface) and the opposite surface (back surface) ) Is plated with Ni, and when the plate surface is viewed from the vertical direction, the direction parallel to a certain end surface is defined as the X direction, and the direction perpendicular thereto is defined as the Y direction, from each end surface facing the X direction. The positions on the plate surface at a distance of 4 mm and in the center in the Y direction are X ₁ and X ₂ , respectively, and the positions on the plate surface at a distance of 4 mm from the respective end surfaces facing in the Y direction are Y ₁ respectively. , Y ₂ , where P is the position on the plate surface at the intersection of the straight line X ₁ -X ₂ and the straight line Y ₁ -Y ₂ , regarding the Ni plating thickness, A heat sink to fill is provided.
ΔA ≦ 2.0μm (1)
Here, ΔA (μm) is a difference (absolute value) between the maximum value and the minimum value of the Ni plating thickness at each position of X ₁ , X ₂ , Y ₁ , Y ₂ , and P.

It is preferable to adjust so that the average value of the Ni plating thickness at each position of X ₁ , X ₂ , Y ₁ , Y ₂ , and P is in the range of 0.1 to 10 μm on the substrate mounting surface or further on the back surface. . Moreover, the heat sink which has an end surface to which Ni plating is not given is provided.

  Such a heat sink can be realized by a manufacturing method having a step of press punching a strip of copper or copper alloy on which Ni plating is applied, or a step of further warping. The press punching step and the warping step are performed by a progressive die press, and the material is transferred in the progressive die by lifting the material with a side guide provided in the die. If the contact between the material and the lower mold during transportation is avoided, generation of scratches on the Ni plating surface during pressing can be suppressed.

  Furthermore, according to the manufacturing method having a step of drying quickly after degreasing the punched plate material using an alkali, alcohol or hydrocarbon-based cleaning liquid after the press punching step or the warping step, for example, Dissipation of the exposed end surface of the substrate due to oxidation, and heat dissipation with good solder wettability, such that when the substrate mounting surface is soldered using a rosin flux, the solder wetting area is 95% or more. A board can be provided.

  According to the present invention, it is possible to stably prevent the formation of voids when using Pb-free solder and the decrease in solder wettability due to the use of Pb-containing solder with a smaller amount of Ni adhesion. Also, keeping the amount of Ni deposited with low thermal conductivity from becoming excessive leads to improved heat dissipation. Furthermore, the heat dissipation plate having the end face not subjected to Ni plating also improved the bondability with the power module resin case. By saving the amount of Ni deposited and being able to apply Ni plating using a continuous plating line, the manufacturing cost can also be reduced, and the remaining processing oil can be properly washed after press punching after plating. It is possible to maintain good solder wettability.

  In addition, it is possible to reduce the amount of warpage fluctuation immediately after pressing by applying Ni plating that causes fluctuation of the warpage amount before the warping process or by using a heat sink with reduced variation in Ni plating thickness. It becomes. Generation | occurrence | production of the harmful damage | wound which arises when pressing the strip | belt material of board thickness 1-5mm is also suppressed. By appropriately cleaning the processing oil remaining after the press punching, it is possible to maintain good solder wettability and to prevent discoloration even at the punched end surface without Ni plating.

FIG. 2 schematically shows the Ni plating thickness measurement position on the plate surface of the power module heat sink. The inventors have investigated the Ni plating thickness of the surface in detail in a conventional heat sink manufactured by applying electric Ni plating after stamping. As a result, it has been found that a difference in plating thickness tends to occur particularly between the central portion of the heat sink and the vicinity of the end face. That is, on both the substrate mounting surface and the back surface, the difference in plating thickness between the center position P and the end surface vicinity positions X ₁ , X ₂ , Y ₁ , Y ₂ shown in FIG. 2 tends to increase. Therefore, it is important to reduce these differences.

The positions of X ₁ , X ₂ , Y ₁ , and Y _{2 on} the plate surface in the vicinity of the end surface are 4 mm from the respective end surfaces, and for X ₁ and X ₂ , the position in the center in the Y direction, Y ₁ And Y ₂ are set at the center in the X direction. In addition, a hole may be partially made in a heat sink according to a use, or the processus | protrusion for ensuring a solder layer thickness may be formed. If the above position hits a processing part such as a hole or a projection, the position on the plate surface at a distance of 4 mm from the end surface in the vicinity of the processing part (approximately within 4 mm) is used as the measurement position instead. Good.

The position of the center P on the plate surface is set at the intersection of the straight line X ₁ -X ₂ (L _{X in} FIG. 2) and the straight line Y ₁ -Y ₂ (L _{Y in} FIG. 2). If the position of P hits a processing site such as a hole or a protrusion, the position on the plate surface in the vicinity of the processing site (approximately within 4 mm) may be used as the measurement position instead.
By measuring the Ni plating thickness for the positions X ₁ , X ₂ , Y ₁ , Y ₂ , and P on the plate surface set in this way, the variation of the Ni plating thickness as the heat sink should be satisfactorily evaluated. Can do.

According to the inventor's detailed investigation, the Ni plating thickness is measured at the measurement position thus determined, and the Ni plating thickness at each of the obtained measurement points satisfies the formula (1). On the substrate mounting surface, generation of voids in the solder layer due to variations in the Ni plating thickness and solder wettability are stably prevented. In addition, on the back surface, heat dissipation is also uniformed by uniformizing the adhesion amount of Ni having a lower thermal conductivity than copper or copper alloy. Furthermore, it is not necessary to increase the amount of Ni adhesion excessively with respect to the required Ni plating thickness on both the substrate mounting surface and the back surface, and material costs can be reduced and heat dissipation can be improved.
Although electroless plating can easily control the thickness variation, the cost and plating stress are large, and it is preferable to apply electroplating in the present invention.

The Ni plating thickness is measured by a fluorescent X-ray film thickness measuring device, an electrolytic film thickness measuring device, or cross-sectional observation, as will be described later. _{Incidentally, X 1, X 2, Y} 1, Y 2, it is desirable that the average value of the Ni plating thickness at each position P is adjusted to the range of 0.1 to 10 [mu] m. If the Ni plating thickness is too thin, Cu diffusion cannot be sufficiently suppressed, and if it is excessively thick, the cost increases, and it becomes increasingly difficult to suppress the variation in plating thickness within the scope of the present invention. The average value of the Ni plating thickness is more preferably 3 to 7 μm.

  Such highly uniform Ni plating can be realized by a method in which Ni plating is performed in advance in a “strip” state before press punching. In particular, Ni plating can be applied to both sides of a copper or copper alloy strip, and each strip can be passed through a continuous electroplating line capable of independent plating thickness control on each side. It is also possible to ensure a Ni plating adhesion amount according to the purpose.

  After plating, a heat sink having Ni plating layers formed on the substrate mounting surface and the back surface is constructed through processing steps including press punching. By punching after performing Ni plating first, the heat sink which has an end surface in which Ni plating is not given is obtained. Thereby, bondability with a power module resin case can be improved simultaneously.

  In the pressing process, in addition to punching to separate parts from the strip, protrusions are formed as necessary, or “reverse warping” to offset “warping” caused by thermal expansion in the cooling process during soldering. Can be formed. These processes can be carried out efficiently by using a progressive die.

  After the pressing step, it is desirable to perform a step of quickly drying after degreasing the punched plate material using an alkali, alcohol or hydrocarbon-based cleaning liquid. Thereby, the discoloration resistance of the end surface and good solder wettability can be secured stably. The alkali here is a substance such as caustic soda, caustic potash or inorganic soda salt. Examples of the hydrocarbon-based substance include paraffin, acetone, dichloromethane and the like.

  The heat sink of the present invention uses copper (purity 99.96% or more) or copper alloy having a thickness of 1 to 5 mm. Examples of the copper alloy include Cu—Fe—P and Cu—Sn. When forming a protrusion by attaching a ring-shaped depression with a processing pin and raising the protrusion at the center, a plate material adjusted to a hardness of about 70 to 200, preferably about 80 to 150, is used for press molding. It is desirable. In addition, the heat sink is often rectangular, but at least one side is preferably 50 mm or longer, and more than 100 mm, and the effects of the present invention can be used well.

When a semiconductor substrate is mounted on the obtained heat sink of the present invention, Sn-Pb solder that is usually used, or Pb-free solder such as Sn-3.5Ag and Sn-5Sb can be applied. For example, a predetermined amount of flux and solder are placed on the plate surface, and a plurality of semiconductor substrates are placed thereon, and then placed in a furnace (eg, N ₂ + H ₂ atmosphere) at a temperature approximately 50 to 100 ° C. higher than the melting point of the solder. After the solder is sufficiently melted, take it out of the furnace and let it cool.

A strip of a Cu-0.07Fe-0.02P alloy (thickness of about 25% after annealing) having a thickness of 3 mm and a width of 235 mm was subjected to continuous electric Ni after the surface was alkali and electrolytically degreased. Ni plating was performed on both surfaces by passing through a plating line. Conditions were set so that the average amount of Ni plating adhered to both surfaces was 3.5 μm.
On the other hand, as a comparative material, a strip material different from the above was prepared only in that Ni plating was not applied.

  Using these strips, “projection formation”, “punching”, and “reverse warp formation” were performed using a progressive die, and about 500 heat sinks each having a size of 100 × 70 mm were manufactured. A total of 12 protrusions having a height of about 150 μm are formed by attaching a ring-shaped depression with a processing pin at the semiconductor substrate mounting position on the substrate mounting surface and raising the protrusion at the center, and the reverse warping occurs immediately after pressing. The mold and pressing conditions were set so that the warpage amount was 300 μm in the X direction and 200 μm in the Y direction. Moreover, about the thing using a comparative material, Ni plating was given by the conventional method by batch processing after that.

  The amount of warpage can be measured with a dial gauge, laser displacement meter, contact type surface shape measuring instrument, etc. on the surface of the measured material (plate-shaped part) placed on a horizontal plane. Is preferable. “Measuring along a straight line” means measuring a straight line when the surface of the material to be measured placed on a horizontal plane is viewed from the vertical direction. The value of the amount of warpage is the difference between the maximum value and the minimum value within the measurement range of the relative height of the surface with respect to the reference horizontal plane. Here, measurement was performed using a dial gauge.

  In addition, the product after press working of the present invention is subjected to ultrasonic cleaning for 5 minutes with a hydrocarbon-based cleaning liquid (specifically, containing 98.5% normal paraffin in a solvent), and then quickly blower Drying was performed.

Ten samples are randomly extracted from the obtained heat sink, and the positions on the plate surfaces a to e and f to j shown in FIG. 1 (P, Y ₁ , Y ₂ , X ₁ , respectively) Ni plating thickness at X ₂ to the corresponding) was measured by a fluorescent X-ray film thickness meter. Specifically, fluorescent X-rays reflected from the surface by irradiating each measurement position with X-rays are detected by a detector, and a calibration curve prepared in advance with a standard Ni thin film sample and detected X-ray intensity. From this, the Ni plating film thickness was calculated.

As a result, all the products of the present invention manufactured by punching after Ni plating satisfy the above formula (1) on both the substrate mounting surface and the back surface, and the variation in plating thickness is stable. And it was very small. On the other hand, in the conventional product manufactured by performing Ni plating after pressing using a comparative material, all samples did not satisfy the above formula (1) on at least one of the substrate mounting surface and the back surface, and the Ni plating thickness The variation was large.
Table 1 illustrates Ni plating thickness measurement results for a certain sample.

  As can be seen from the data in Table 1, it can be seen that the conventional product has a particularly large difference in plating thickness between the center position and the end face position. The other nine samples are almost similar to this.

Next, 10 samples each were extracted from the product of the present invention and the conventional product, and the amount of warpage variation from immediately after the warpage to the amount of warpage in a steady state was investigated.
For the products of the present invention, after applying warp to the press, warping was performed with target values of 300 μm in the X direction and 200 μm in the Y direction, and then the cleaning process was performed. By measuring the amount of warpage, it was confirmed that the amount of warpage no longer fluctuated, that is, the amount of warpage was in a steady state. In addition, discoloration etc. were not observed on the end surface of the left heat sink.
In addition, the conventional product using the comparative material is warped at the target value of 300 μm in the X direction and 200 μm in the Y direction immediately after the warping of the press, and then the amount of warpage is measured after performing the electric Ni plating by batch processing, and further 7 days The amount of warpage was measured by leaving it alone, and it was confirmed that the amount of warpage was in a steady state.

When the warpage amount in the X direction is δ _X0 , the warpage amount in the Y direction is δ _Y0 and the warpage amount in the Y direction is δ _Y0 immediately after the warpage is applied with the target values of 300 μm in the X direction and 200 μm in the Y direction. when the warp amount in the X direction and [delta] _X1, Y direction warpage [delta] _Y1 by actual measurement, the warp variation was calculated by the following equation.
X direction warpage variation (%) = | δ _X0 −δ _X1 | / δ _X0 × 100
Y direction warpage variation (%) = | δ _Y0 −δ _Y1 | / δ _Y0 × 100
As a result, in the present invention, the amount of warpage fluctuation in the X direction and the Y direction was 7% or less in all samples. In contrast, in the conventional product using the comparative material, the amount of warpage fluctuation in at least one of the X direction and the Y direction exceeded 7% in three samples.

Next, a semiconductor substrate having a thickness of about 0.3 mm with a copper pattern formed on the surface of an aluminum nitride substrate, the solder joint surface on the heat sink side (the surface corresponding to the conductor layer 6 in FIG. 1) is 70 ×. A 50 mm one was prepared. This was mounted by soldering on 100 heat sinks each randomly extracted from the heat sink of the present invention and the heat sink of the conventional product. For the solder, prepare Sn-3.5Ag alloy paste containing rosin-based flux, apply an appropriate amount according to the thickness of the solder layer to the specified position of the heat sink by a normal method, and put the semiconductor substrate on it It was placed in a furnace at 300 ° C. and N ₂ + H ₂ atmosphere, and after about 10 minutes, it was taken out of the furnace and allowed to cool.

  Of the 50 samples that have been soldered, an ultrasonic photograph of the board joint is taken for each of the 50 samples, and the resulting photograph is subjected to image analysis, so that voids are generated in the solder layer and the solder wettability is reduced. It was investigated whether or not there were places where the solder wetted area was 95% or less (hereinafter referred to as “solder defects”). As a result, no solder defects were observed in all 50 samples using the heat sink of the present invention. On the other hand, in the sample using the heat sink of the conventional product, 5 out of 50 solder defects (including those practically acceptable) were found.

  Further, an operation was attempted in which the remaining 50 samples among the samples after solder bonding were inserted into a resin case of a predetermined power module. As a result, since the heat sink of the product of the present invention is not Ni-plated on the end face, it is stable and strong between the end face and the resin case compared to the heat sink of the conventional product in which the end face is also Ni-plated. Bonding force was obtained.

Sectional drawing which showed typically the example of a structure of the semiconductor module which mounted the semiconductor substrate on the heat sink. The figure which showed typically the plating thickness measurement location in the board surface of a heat sink.

Claims (10)

In a heat sink made of copper or copper alloy having a thickness of 1 to 5 mm for mounting a semiconductor substrate by solder bonding, a surface on which the semiconductor substrate is mounted (hereinafter referred to as “substrate mounting surface”) and an opposite surface (hereinafter referred to as “substrate mounting surface”). When the plate surface is viewed from the vertical direction, the direction parallel to a certain end surface is defined as the X direction, and the direction perpendicular to the Y direction is defined as the Y direction. X1 and X2 are positions on the plate surface at a distance of 4 mm from the end surface of the Y-axis, and Y1 is a position on the plate surface at a distance of 4 mm from each end surface facing the Y-direction and at the center in the Y-direction. , Y2, and a position on the plate surface at the intersection of the straight line X1-X2 and the straight line Y1-Y2, where P is the heat sink that satisfies at least the following expression (1) with respect to the Ni plating thickness.
ΔA ≦ 2.0μm (1)
Here, ΔA (μm) is a difference between the maximum value and the minimum value of the Ni plating thickness at each position of X1, X2, Y1, Y2, and P.   The heat sink according to claim 1, wherein both the substrate mounting surface and the back surface satisfy the formula (1). _2. The heat sink according to claim 1, wherein the average value of the Ni plating thickness at each position of X ₁ , X ₂ , Y ₁ , Y ₂ , and P is in the range of 0.1 to 10 μm on the substrate mounting surface. The heat sink according to claim 2, wherein the average value of the Ni plating thickness at each position of X ₁ , X ₂ , Y ₁ , Y ₂ , and P is in the range of 0.1 to 10 µm on both the substrate mounting surface and the back surface.   The heat sink in any one of Claims 1-4 which has an end surface to which Ni plating is not given.   The heat sink according to any one of claims 1 to 5, wherein when the board mounting surface is soldered using a rosin flux, the solder wetted area becomes 95% or more.   The manufacturing method of the heat sink in any one of Claims 1-6 which has the process of carrying out the press punching of the strip of copper or copper alloy to which Ni plating was given.   The manufacturing method of the heat sink of Claim 7 which has the process of giving warp after the process of press punching.   The press punching step and the warping step are performed by a progressive die press, and the material is transferred in the progressive die by lifting the material with a side guide provided in the die. The manufacturing method of the heat sink of Claim 8 which avoids the contact at the time of the transfer of material and a lower mold | type.   10. The method according to claim 7, further comprising a step of degreasing the punched plate material using an alkali, alcohol, or hydrocarbon-based cleaning liquid and then quickly drying it after the press punching step or the warping step. The manufacturing method of the heat sink of description.

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